This application claims priority under 35 U.S.C. xc2xa7119 to European Patent Application No. 01310739.6 filled on Dec. 20, 2001.
This invention relates to controlling rise and fall times of a digital waveform, for example as produced by a line driver.
Often in the art, it is thought that digital waveforms should have rise (and fall) times that are as short as possible, and indeed in the idealised form of these waveforms that is often considered those times are taken to be zero. Short rise times mean, however that the waveform has large high frequency components and in some applications these can cause malfunctions in circuits receiving the waveforms. Also having large high frequency components can be undesirable when transmitting the waveform, particularly for example when the high frequencies travel at a different speed.
It is an object of the invention to produce a digital waveform with controlled rise and fall times. A further object is to make those times selectable.
A known way to control an edge in an output is described in the applicant""s U.S. Pat. No. 6,215,816. That arrangement provides a local area network physical layer interface device for connection to an unshielded twisted pair cable. Each edge in an output is divided into a series of sub edges each provided by separate component drivers, each triggered in turn with small delays between them. Thus, the overall rise/fall time (across all the sub edges) is controllable via the delay between each of the sub edges. The device includes synthesised rise time control for reduced electromagnetic interference, (which again is produced by high frequency components). The twisted pair is driven by component drivers that are triggered to give their contribution to the signal edge by respective taps from a ring oscillator.